Help, can't get r23 pot to adjust

[Young Grasshopper]

Hey Carl, I can't seem to zero out pins 2 and 3 of ic8. Everything up to this point has been perfect. Any clues as to this. What should I look for. I think all my values are right. I'm following your directions but am clueless. Voltage across 2 and 3 is 1.12 and adjusting r23 changes voltage from 1.120 to 1.123. I can adjust r18 to zero out tp4 but nothing with r23. I'm a newbie. Thanks again.